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do you have brake fluid when the brake pedal is pushed and the bleeder screw is opened. if so then it could be a seized caliper piston, which indicates the caliper needs to be replaced and system bleed. if not check for fluid before the brake flex hose again with someone pushing on the pedal. if you have pressure then the flex hose has an internal separation and needs replacing. if not then you will have to find were your loosing pressure between there and the master cylinder.
Usually a hard brake pedal is the brake booster. Since you replaced that it is out f the picture (unless the replacement was bad). This issue of the pedal going to the floor is troublesome and potentially dangerous. If it is not doing it all the time, it probably soon will. What most likely occurred, and this happens quite often, is that after having had the hoses and calipers replaced, it was necessary to bleed the brake system of air in order to get proper brake operation. This requires depressing the brake pedal numerous times to the floor while purging air from the system. There is a potential problem in this and that problem is that the o-rings in the master cylinder can be damaged by the bleeding process, because the normal stroke of the master cylinder is much shorter than the one used during bleeding. This sometimes results in o-ring damage because the master cylinder bore may have worn a ridge in it where the normal stroke ends. The repeated passing of the o-rings over the ridge will damage them and can cause them to fail. When this happens the pedal will drop to the floor. You really can't blame the mechanic for this but it most likely happened as a result of the work done. Have the master cylinder replaced ASAP. I wouldn't recommend using the vehicle at all in the meantime.. it is a pedal-press away form total brake failure at any given moment.
Time to redo the bleeding of the system again...starting with the master cylinder...you do not have to remove the master to re-bleed it. disconnect both brake lines from the master, and start again by pushing pedal to floor, and rebleed each line at the master...this is critical to work properly...you mentioned new master cylinder, if remanufactured I would return it. As noted above, after confirming the master is fully bled...rebleed each caliper beginning with the rear, farthest away from the master...you must have "air" in the brake lines. Hope this helps.
The brake system bleeding procedure differs for ABS and non-ABS
vehicles. The following procedure pertains only to non-ABS vehicles. For
details on bleeding ABS equipped vehicles, refer to the ABS procedures
later in this section.
Make sure the master cylinder contains clean DOT 3 brake fluid at all times during the procedure.
The master cylinder must be bled first if it is suspected of containing air. Bleed the master cylinder as follows:
Position a container under the master cylinder to catch the brake fluid.
Loosen the left front brake line (front upper port) at the master cylinder and allow the fluid to flow from the front port.
Connect the line and tighten to 24 ft. lbs. (32 Nm).
Have an assistant depress the brake pedal slowly one time and hold
it down, while you loosen the front line to expel air from the master
cylinder. Tighten the line, then release the brake pedal. Repeat until
all air is removed from the master cylinder.
Tighten the brake line to 24 ft. lbs. (32 Nm) when finished.
Repeat these steps for the right front brake line (rear upper port) at the master cylinder.
Do not allow brake fluid to spill on or come in contact with the
vehicle' finish, as it will remove the paint. In case of a spill,
immediately flush the area with water.
If a single line or fitting was the only hydraulic line
disconnected, then only the caliper(s) or wheel cylinder(s) affected by
that line must be bled. If the master cylinder required bleeding, then
all calipers and wheel cylinders must be bled in the proper sequence:
Bleed the individual calipers or wheel cylinders as follows:
Place a suitable wrench over the bleeder screw and attach a clear plastic hose over the screw end.
Submerge the other end in a transparent container of brake fluid.
Loosen the bleed screw, then have an assistant apply the brake
pedal slowly and hold it down. Close the bleed screw, then release the
brake pedal. Repeat the sequence until all air is expelled from the
caliper or cylinder.
When finished, tighten the bleed screw to 97 inch lbs. (11 Nm) for the front, or 66 inch lbs. (7.5 Nm) for the rear.
Check the pedal for a hard feeling with the engine not running. If
the pedal is soft, repeat the bleeding procedure until a firm pedal is
Fig. 1: Loosen the front brake line in order to bleed the master cylinder
Fig. 2: Connect a bleed hose from the bleed valve on the front caliper to a jar of brake fluid
Fig. 3: Always follow the lettered sequence when bleeding the hydraulic brake system
Hope this helps to solve it; remember to rate this answer.
DId you bleed the system after you changed the caliper? If the Master cylinder lost it's fluid while you were changing the caliper, then there is air in the system and you need to bleed the entire system. start with the REAR RIGHT, then the REAR LEFT and then the FRONT RIGHT and last the FRONT LEFT. make sure the master cylinder NEVER runs low on fluid and DO NOT re-use any fluid that you get out of the calipers.
This may sound dum but its been done a few times and is a simple mistake , the bleeder screw is up top, with the caliper on the bleeder screw should be up top, if not then calipers are on the wrong side, it sounds like air in the system, also you pumped the pedal up to push out the piston so pads hit rotor after bleeding,i dont see anything else, if the pedal is good with pinched lines at caliper then it got to be calipers are wrong or on the wrong side and the bleeder is not atop caliper,also you bleed the rear also? these trucks and cars use left caliper and rt rear on one system of the master and rt front and left rear on one side, let me know about the caliper, heres a picture to show you what i mean. hope this helps
your power brake booster is blown out
the seal has a leak in it
run the truck for 3 minutes and shut it off. with the windows up listen carefully while you gently apply the brakes
listen for a hissing sound of air leaking.
the sound of these leaking is a very soft sound
hi from the uk have had this problem on a customers car b4 who had replaced frot calipers and pads and pedal went to floor on inspection of caliper i found that the steel spring clip that fits into two holes in the front face of caliper and must also locate behind the caliper carrierbehind two lugs was fitted incorrectly it had been fitted behind the outer pad resulting in when brake pedal off ?the caliper being floating type as it is known because it is fixed by two screw pins/bolts and moves on these pins/bolts as pads wear ? what happens is the spring fitted wrongly ? actually pushes against the outer pad and as a result the caliper piston is pushed back into its cylinder slightly as it moves on the locating pins/bolts resulting in pedal going to floor on 1st application but if pumped ? some brake pedal force is felt but is lost again when pedal released ? so try this ? remove the steel clips from both front calipers that retain caliper to carriers then press brake/pump pedal ? if as suggested above works? your pedal should pump up and remain ok when you release and still be firm when re applied ? all above is what i found on ford ford but many other models use similar calipers and spring locating recheck how this spring should locate hope this helps ? good luck
Im going to assume that a mechanic has already checked for the most simple causes like making sure the brakes have been bled. If the abs module is faulty it will default to just a no abs state, but that wont cause the pedal to go all the way to the floor. I agree that replacing the caliper is probably gonna fix the problem, but I might try to bleed the brakes first and see if that solves it first...